We propose to evaluate the feasibility of using immobilized soil bioreactors (ISBRs) for detoxification of the naphthenic acids (NAs) found in oil sands tailing waters. Microbial communities bound to NAs contaminated sediment will be entrapped in an inexpensive, woven fiber support of two ISBRs. These microbes will form the basis of a biofilm that will develop in the presence of a defined mineral salts medium and a commercially available source of NAs (Merichem) as sole carbon and energy source. The resulting large population of NAs degraders should allow high degradation rates with minimal loss of the degrading population from perturbations in operating conditions or influent composition. The ISBR biofilms will be challenged with an increasingly more complex NAs source - extracted NAs, then an oilsand wastewater. All media will be kept at pH 8 to 8.5 (typical pH of tailings water and of the Athabasca River). This will also keep NAs fully ionized, avoiding solubility and bioavailability issues and make it easier to quantify degradation using established GC/MS and HPLC/QTOF-MS methods. Continual monitoring of the gas phase by O2 and CO2 analyzers in the ISBR gas recycle line will allow detection of the biodegradation of even nanomolar quantities of NAs. Parameters affecting degradation kinetics (such as NAs, NaCl and dissolved oxygen concentrations, NAs composition, and temperature) will be studied. Toxicity (Microtox) assays will be correlated with NAs degradation and with the biofilm and planktonic microbial community in the ISBR. These bacteria will be identified based on 16S rDNA gene sequences. Attempts will be made to isolate organisms implicated in NAs degradation and identify their role in the community. If NAs degradation and detoxification are successful, recommendations will be made for potential full-scale treatment in which the ISBR may be integrated with other technologies. Training will be provided for 2 PhD and 4 undergraduate students in a challenging, multidisciplinary project. This expertise can be applied to the biological treatment of many other contaminants. ISBR technology has the potential to provide an innovative solution to the challenge of economically detoxifying large volumes of oilsands process water and knowledge arising from this project will contribute to further understanding the biodegradation of its most toxic component.